A New Class of Black Holes?

A New Class of Black Holes?

Astronomers may have discovered a new type of middle-weight
black hole in the centers of some galaxies.

FROM A NASA HQ PRESS RELEASE

Apr. 13, 1999:
The field of black holes, formerly dominated by
heavyweights packing the gravitational punch of a billion Suns and
lightweights just a few times heavier than our Sun, now has a new
contender -- a just-discovered mysterious class of "middleweight"
black holes, weighing in at 100 to 10,000 Suns.

Astronomers at NASA and Carnegie Mellon University have
independently found evidence for the new type of black holes in
spiral-shaped galaxies throughout the Universe. The newfound
black holes, formed by an unknown process, are 100 to 10,000 times
as massive as the Sun, yet each occupies less space than the Moon.

Right: M82 is a nearby galaxy now thought to harbor
a middling-weight black hole in its nucleus. It is a member of a
group of galaxies dominated by itself, M81, and NGC 3077. M82 is thought by some to be limping
away from a close encounter with M81. This galactic collision might have stirred up the inner stars and
gas in M82, causing the unusual dark lanes of dust visible in the above photograph.
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A black hole is a region of space where the force of
gravity is so powerful that nothing, not even light, can escape
its pull. Until now, scientists knew about two types of black
holes: stellar and supermassive. Stellar black holes are the
remains of dead stars several times heavier than the Sun,
compressed to a diameter of a few miles or less. Supermassive
black holes have mind-boggling masses of one million to one
billion Suns and may have formed in the early universe from giant
gas clouds or from the collapse of clusters of immense numbers of
stars.

The astronomers identified the new class of black holes
through X-ray light, the final cries of energy emitted from gas
and particles spiraling into a black hole. The discovery will be
announced today at the meeting of the High Energy Astrophysics
Division of the American Astronomical Society in Charleston, SC.

Dr. Edward Colbert and Dr. Richard Mushotzky, astronomers
at NASA's Goddard Space Flight Center, Greenbelt, MD, first saw
hints of the new class of black holes while studying X-rays from
39 relatively nearby galaxies. Dr. Andrew Ptak and Dr. Richard
Griffiths at Carnegie Mellon University studied X-ray light from a
galaxy not included in Colbert and Mushotzky's set, galaxy M82.
Both teams found unique X-ray light indicative of a new black hole
class. The results from both teams will be published in the
Astrophysical Journal and the Astrophysical Journal Letters,
respectively.

"Our intent was to understand what was producing an unusual
class of X-ray luminosities near the centers of many galaxies,"
said Colbert. "With data from the Einstein satellite from the
1970s, we couldn't determine whether they had features associated
with supermassive black holes or stellar black holes. So we took
a fresh look with newer data."

Colbert and Mushotzky found telltale clues for a new type
of black hole in the spectrum, or colors, of the invisible X-ray
light. Such colors are judged by comparing the intensity of X-
rays with shorter wavelengths to those with longer wavelengths,
just as blue skylight is mostly composed of shorter wavelengths
than the light from a red sunset.

Supermassive black holes are thought to power a phenomenon
called Active Galactic Nuclei, which are extremely compact and
energetic objects seen in the core of one percent of all galaxies
and are typically very bright X-ray sources. The luminosities
that Colbert and Mushotzky analyzed have colors different from
those found in Active Galactic Nuclei, suggesting the source is
something other than a typical supermassive black hole.

Right: NGC 4945 is a spiral galaxy in the
Centaurus Group of galaxies, located only six times further away than the prominent Andromeda
Galaxy.
NGC 4945 is thought to be quite similar to our own Milky
Way Galaxy. X-ray observations reveal, however, that NGC 4945 has an unusual, energetic, Seyfert 2
nucleus that might house a large black hole.
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Ptak and Griffiths acted on the belief among astronomers
that black holes of various sizes must exist and likely reside in
"irregular" galaxies (galaxies not spiral or elliptical in shape).
M82 is one such galaxy, called a starburst galaxy because of the
high rate of star formation found inside. Such a scenario leads
to a higher rate of supernovae, or star explosions, the precursor
of stellar black holes.

"Millions of black holes and neutron stars have formed in
M82 over the last 10 million years," Ptak said. "Now, we are
noticing that some of these may be coalescing into a larger-mass
black hole." Ptak said this is the most viable current theory for
intermediate black hole formation. Colbert also said the
intermediate class suggested by his and Mushotzky's observations
might be formed by "the continual merging of stellar black holes."
In other words, stellar black holes that approach each other too
closely under certain circumstances can merge to form a more
massive single black hole. This process might build objects that
produce the peculiar colors of these X-ray glows.

Ptak and Griffiths used data from the Japan-U.S. Advanced
Satellite for Cosmology and Astrophysics (ASCA). Colbert and
Mushotzky used data from the German/US/UK ROSAT satellite and
ASCA. Japanese researchers led by Dr. Tsunefumi Mizuno at the
University of Tokyo have reported results similar to Colbert and
Mushotzky's. Dr. Takehishi Go Tsuru at Kyoto University and
colleagues have found data supporting Ptak and Griffiths' work.
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